Our invention relates to a combining and filter circuit for a phase locked loop, and particularly to such a circuit that combines phase detector and acquisition circuit signals, and filters those signals for application to a voltage controlled oscillator in a phase locked loop.
Phase locked loops using a reference oscillator and a voltage controlled oscillator usually include an acquisition circuit to provide signals that bring the voltage controlled oscillator frequency within the range of the phase detector circuit. After the voltage controlled oscillator frequency is within the range of the phase detector circuit, signals from the phase detector circuit can then control the frequency of the voltage controlled oscillator. Where an acquisition circuit is used in addition to a phase detector circuit, it is necessary to provide an arrangement that applies the signals from the two circuits to the voltage controlled oscillator.
Accordingly, a primary object of our invention is to provide a new and improved circuit that combines signals from an acquisition circuit and from a phase detector circuit for application to a voltage controlled oscillator in a phase locked loop.
A general object of our invention is to provide a new and improved signal combining circuit for use in a phase locked loop.
In phase locked loops, filtering is needed or desired to attenuate the reference frequency signals so that the voltage controlled oscillator does not produce spurs indicative of those reference frequencies. Low pass filtering is also needed or desired to provide relatively stable control signals for the voltage controlled oscillator.
Accordingly, another object of our invention is to provide a new and improved filter circuit for use in a phase locked loop.
A relatively specific object of our invention is to provide a new and improved filter circuit that attenuates the reference frequency signals in a phase locked loop, and that provides low pass filtering of signals applied to a voltage controlled oscillator.
Another object of our invention is to provide a new and improved circuit that combines signals from an acquisition circuit and from a phase detector circuit, and that filters these signals for application to a voltage controlled oscillator in a phase locked loop.
Another object of our invention is to provide a new and improved signal combining circuit that attenuates reference frequency signals and provides low pass filtering of the combined signals for application to the voltage controlled oscillator in a phase locked loop.